1. Field of the Invention
The present invention relates to an exhaust gas recirculation control system for an internal combustion engine for use in automebiles or the like and more particularly, to the system being capable of obtaining the amount of exhaust gas recirculation which accurately depends on the amount of inlet air.
2. Description of the Prior Art
Generally, in order to decrease the amount of nitrogen oxides in exhaust gas from automobiles, there is available a well known effective measure in which the exhaust gas is partly subjected to recirculation into the inlet channel to thereby lower the maximum temperature at the time of combustion. Since the exhaust gas recirculation is, however, carried out at the cost of low operational efficiency and degradation in fuel comsuption rate, it is necessary to adjust and attain an optimum exhaust gas recirculation rate (hereinafter referred to as EGR rate) which meets operational conditions.
An approach is made today to a control system wherein a Venturi negative pressure in the carburetor is detected to obtain an optimum EGR rate in accordance with the amount of inlet air. In this respect, one may refer to Japanese Patent Application Laid-Open No. 4423/75 issued on Jan. 17, 1975. According to this Japanese Patent Application, while make-and-break of an exhaust gas recirculation control valve (hereinafter termed EGR control valve) is controlled in accordance with Venturi negative pressure and engine speed, the operation of the EGR control valve is feedback-controlled by monitoring the amount of exhaust gas recirculation per se.
According to another approach such as Japanese Patent Application Laid-Open No. 77924/77 issued on June 30, 1977, an orifice is arranged in an exhaust gas recirculation passage to generate a negative pressure in proportion to the amount of exhasut gas recirculation and this negative pressure is applied to a valve mechanism which generates a pressure signal for feedback controlling an EGR control valve. The valve mechanism generating the controlling pressure uses a diaphragm mounted in a chamber to which the negative pressure in the exhaust gas recirculation passage is introduced. The diaphragm used in such a valve mechanism is usually made from a rubber sheet and chemically sensitive to SO.sub.2 and ozone contained in the exhaust gas, being liable to change its state. In addition, due to frequent repetition of vibratory deformations during the operation of automobile, this diaphragm gradually becomes worn, giving rise to its inaccurate operation and degradation in its controlling accuracy. This leads to necessity of exchanging the valve mechanism which can be accomplished only with high skill and at a high cost.